Does anybody know this? ?

h = 3a + 28.6
a pediatrician uses the model above to estimate the
height h of a boy, in inches, in terms of the boy's
age a, in years, between the ages of 2 and 5. based
on the model, what is the estimated increase, in
inches, of a boy's height each year?
a) 3
b) 5.7
c) 9.5
d) 14.3